1. Field of the Invention
The present invention relates to an improvement in structure of a monolithic ceramic capacitor in which internal electrodes are formed by defining gaps in a ceramic member to inject molten lead or lead alloy into the gaps.
2. Description of the Prior Art
FIG. 2 is a sectional view showing an example of a conventional monolithic ceramic capacitor which has motivated the present invention. A monolithic ceramic capacitor 1 as shown in FIG. 2 comprises a plurality of internal electrodes 3 laminated through a ceramic member 2 and a pair of external electrodes 4 and 5 connected with prescribed ones of the internal electrodes 3 to extract the electrostatic capacity. This monolithic ceramic capacitor 1 is manufactured in the following manner: A plurality of ceramic green sheets in thickness of about 50 to 100 .mu.m are prepared to be printed with paste of carbon powder or a mixture of carbon powder and ceramic powder. Then the plurality of ceramic green sheets are laminated and fired so that the carbon powder is dissipated by combustion to define gaps in portions to be provided with the internal electrodes 3. Thereafter porous Ag-fired layers are formed on end surfaces of the ceramic member 2 to define the external electrodes 4 and 5. The chip thus obtained is dipped under decompression in molten lead of about 330.degree. to 360.degree. C. while pressurizing the space above the molten lead thereby to inject the molten lead into the gaps defined in the ceramic member 2. Then the chip dipped in the molten lead is taken out from the same to be cooled and returned to the atmospheric pressure to form the internal electrodes 3. Thus, the internal electrodes 3 can be formed by lead which is low-priced base metal, whereby the monolithic ceramic capacitor 1 can be obtained at a low cost.
In order to manufacture the aforementioned monolithic ceramic capacitor 1, the Ag layers for forming the external electrodes 4 and 5 are provided in advance of injection of the material for the internal electrodes 3 so as to prevent leakage of the molten lead from the gaps that may be caused when the chip is taken out from the molten lead even if pressure regulation is performed sequentially (decompressed state - pressurized state - atmospheric pressure state), by previously forming the porous external electrodes 4 and 5. However, when the chip is dipped in the molten lead, porous Ag forming the external electrodes 4 and 5 is dissolved in the molten metal whereby the external electrodes 4 and 5 are dissipated or eroded and cannot effectively prevent the aforementioned leakage of the molten lead from the gaps. Further, lead reacts with Ag to define a Pb-Ag alloy layer to cause difficulty in soldering in a next step of mounting the chip on a substrate or connecting the same with lead wires.
An example of the conventional monolithic ceramic capacitor in the aforementioned structure is disclosed in U.S. Pat. No. 4,071,880 issued on Jan. 31, 1978 to T. C. Rutt. This prior art example shows penetrable barriers made of palladium-silver or palladium-gold for forming external electrodes. Consequently, the penetrable barriers may be easily eroded by the molten lead, while the cost for forming the external electrodes is increased.